Process of magnesium production and furnace equipment therefor



Dec. 5, 1944. w. A. DARRAH 2,364,195

PROCESS OF MAGNESIUM PRODUCTION AND FURNACE EQUIPMENT THEREFOR Filed June 22, 1942 5 Sheets-Sheet l fmentar:

Dec. 5, 1944; w. A. DARRAH 2,364,195

PROCESS OF MAGNESIUM PRODUCTION AND FURNACE EQUIPMENT THEREFOR Filed June 22, 1942 3 Sheets-Sheet 2 ZIM OQSQMQ/J Dec. 5, 1944 w. A- DARRAH 2,364,195

PROCESS OF MAGNESIUM PRODUCTION AND FURNACE EQUIPMENT THEREFOR Filed June 22, 1942 3 Sheets-Sheet 3 In venor;

Patented Dec. 5, 1944 UNITED STATES PATENT orr ca PROCESS OF MAGNESIUM PRODUCTION AND FURNACE EQUIPMENT THEREFOR 4 Claims.

This invention relates to a process for preparing metallic magnesium and to electrically heated furnaces designed to operate at pressures below the atmosphere and in a confined space from which air is excluded.

The process and equipment may obviously be applied to numerous other heating and distillation problems such for example as the electrothermal reduction ofzinc, arsenic, and numerous other materials.

The description of the process and equipment will be directed primarily to the reduction of metallic magnesium in order to simply illustrate the principles involved.

Some of the objects of this invention are to economically and rapidly produce large quantities of the metallic magnesium. Other objects are to provide continuous or semi-continuous processes for carrying on this operation.

The process and equipment offered has substantially new advantages in that maintenance which is now a substantial factor in magnesium production is largely eliminated, operating costs are reduced, the heat required is delivered at the required point and not transmitted through an alloy shell. Numerous other objects and advantages will be-apparent' from a study of the disclosure, drawings and claims submitted herewith.

Referring to the drawings,

Figure I shows in diagram an end elevation of a typical assembly illustrating one form of my invention arranged for continuous operation.

Figure 11 is a section in plan on the line 11-11 showing one form of my device.

Figure III is a schematic diagram of one form of arrangement of electric circuits and auxiliaries which contribute to' the successful operation pf the process and equipment.

. Figure IV is analternate circuit diagram.

Referring to Figure 1, reference I indicates the furnace equipment used in this process. The

. convenient point and is designed to provide an exit for the volatile materials liberated by the thermal reaction within the furnace. H indicates a cooling chamber provided with an air jacket I! which is designed to permit maintaining the required temperature in the condenser. Air (or water) is circulated through air jacket I! entering by inlet is and passing through blast gate H which is arranged to control the rate of flow of air so that the temperature in the receiving chamber will be suflicient to condense the vapors from the furnace but preferably maintain them in a liquid form. Receiving chamber ll may be either an empty chamber and the con- 15 densate allowed to flow down the inner walls into the discharge hopper H at the bottom or as an alternate construction a rotating scraper I! may be mounted in bearings II and I8 and driven by means of motor l9 and V-belt so as to revolve I go continuously and clean the walls. A housing ii is indicated as surrounding bearings and motor,

to prevent the entrance of air into the rotating system. If the rotating member II is employed it is presumed that the condensate will be collected as a solid dust or powder instead of a liquid although of course the rotary wiper may be utilized in either case.

The rotary wiper may be constructed in any of the known commercial types but is illustrated as comprising a series of arms as for example 22 which carry a scraper bar ilwhich revolves in close contact with the inner wall of the condenser ll.

.A duct 24 at the bottom of the hopper serves asa lock and members 25 and it serve to tightly seal the look from the atmosphere and from the receiver thus making it possible to remove controlled quantities of the product in the receiver as desired.

Figure II shows in plan more details of the furnace design itself.

In Figure 11, l indicates the furnace which is enclosed in an air tight steel shell 21 which for convenience may be cylindrical.

Shell 21 is provided with air tight ends 28 and 29 which mount water cooled housings 30 and II respectively. Shell 211s lined with a layer of insulation indicated by 32 and may be 5 provided with an inner layer of insulating refractory 38 and a final layer-of magnesitc or other highly refractory inert material as indicated by 34. A pair of conducting electrodes SI and 30 are, provided entering the shell through water I cooled eealmembers II and ii. These electrodes may to advantage be formed from graphite or other forms of carbon but I do not wish to be restricted to this material only as numerous other high melting conductors may be employed.

The water cooled seal members 30 and 3| are provided with cooling coils 31 and 38 which carry away the heat conducted through the electrodes and the walls. The space between the electrode and the housing may be filled with an insulating plastic such as tar or bitumin indicated by 39. Bushings of soapstone or porcelain 40 and 4! are provided to insulate the electrodefrom the steel housing. Internal bushings 42 and 43 are provided to guide and support the electrodes in the hot zone.

Each main power electrode is provided with an ignition electrode indicated by numbers 44 and 45. These electrodes are also sealed into a water cooled housing member 45 and 41, the construction of which is substantially identical, except for size, with the construction of the seal members used on the large electrodes. These seal members are provided wtih water cooling coils 48 and-49, sealing compounds 50 and and bushings 52 and 53 all as previously described in the case of the larger electrodes.

It will be noted the lining extends around the entire interior of the furnace chamber and for a portion of the duct leading to the receiving chamber. It is desired to reduce the area of the duct as much as possible in order to avoid the loss of radiant heat from the furnace.

54 indicatesua thermocouple or other temperature registering device which serves through a standard commercial relay (not shown) to control the delivery of heat to the furnace.

55 indicates a duct or port through the insulating lining connecting to a pipe 55 and an electrically operated valve 51. Valve 51 is connected by pipe 58 to storage chamber 59 containing hydrogen or other inert gases. The object of the assembly including automatic control valve 51 and reservoir 59 is to deliver controlled amounts of gas into the furnace as required by operating conditions.

Referring to Figure III, which shows a diagrammatic arrangement of the electrical circuits used, I indicates the furnace, and corresponding parts in the other figures are given corresponding numbers;

50 indicates the secondary or high voltage side of a small high voltage ignition transformer of which 5! is the primary or low voltage side. References 52 and 53 designate a similar transformer.

These transformers are designed to supply a voltage as for example 10,000 volts and are of a construction similar to those used on neon lights, etc. These transformers serve to cause a high voltage discharge which locally heats and ionizes the main power electrodes to permit the arc to start, between 35 and 35.

63A represents the secondary of the main power transformer and 54 represents the primary of the transformer provided with a series of taps as indicated by reference 55.

An excess variable inductance 55 is shown connected into the circuit so as to stabilize the are formed. 51 indicates one winding of a series transformer of which 58 is the other winding. The series transformer serves by means of a relay 59 to actuat solenoid valve 51 thus allowing gas to enter furnace chamber I from the reservoir 59 as required. A short circuiting switch is provided around winding 51 of the series transformers to cut it out of the circuit when not desired.

The method of operation of the device employed in my process consists in applying a high voltage to the gap between the ignition electrodes and the main power electrodes. This causes a small arc to pass across this gap which locally heats the main power electrodes causing them to reach extremely high local tempera ture thus causing a flow of electrons from the hot spots and getting them in readiness to support an arc. During this period of the operation the pressure within the furnace chamber should be maintained at a figure substantially below the atmosphere ranging preferably between 26" and 28" absolute of mercury. This may be obtained by means of pump ll connected to pipe 55 through valve 12.

Having now prepared the power electrodes by localized heating so that they are ready to support an are power may be applied to the primary 54 of the main power transformer. A current will now flow between electrodes 35 and 35. The flow of current will prove to be somewhat unstable as the are formed under these conditions will tend to show a lower resistance with an increased current.

This condition may be stabilized by means of the variable inductance 55 or its electrical equivalent, impedance or resistance.

' It will be obvious that of the well known devices for maintaining a substantially constant current in an unstable circuit may be employed. These well known standard devices include such items as the floating coil transformers, formerly used on series are lighting circuits, the well known induction regulator, and the mere use, of a high inductance in series with the power transformer. Transformers having a higher degree of magnetic leakage are quite desirable for this application.

BI and 82 are relays of standard commercial types which are placed so that they operate when the current flowing through the main arc reaches a predetermined value. In other words they are the equivalent of series relays actuated by a minimum current flow in the main circuit. They serve to open the circuit of transformers 52 and 50 which supply power for the auxiliary are used in starting the main arc.

It will therefore be apparent that these relays provide a, means whereby the igniting arcs may be cut out as soon as the main arc is established.

Other obvious electrical equivalents may be of course employed to accomplish this same result, the important point being that the auxiliary arc is extinguished when current is passing through the main arc.

It will be apparent also with this circuit that if for any reason the main arc is extinguished relays will promptly close the circuits feeding the auxiliar arcs and thus re-establish the circuit between the main electrodes.

Figure IV shows an alternate method of starting the main arc initially. This method utilizes the high frequency high voltage ignition circuit. Similar numbers designate similar portions of the equipment.

In Figure IV 35 and 35 indicate the main electrodes sealed in an air tight manner into container I. A source of power, preferably alternating current, at perhaps 1000 volts is applied between the terminals V. Adjustable inductances ll andli areprovidedinthecircuit between the source of power and the electrodes 3! and 38.

Chokecoilsflandltareprovidedinthecircuit between the power source and the main electrodes. These choke coils should be designed and insulated to provide an extremely high impedance to high frequency circuits but with a negligible impedance to ordinary commercial power circuits as for example those of 60 cycle frequency.

Connected also to main electrodes 35 and I. are terminals of a high frequency circuit which will be described. H indicates the terminals of a high frequency circuit which may be obtained from an oscillating spark or are, from a high frequency generator. vacuum tube oscillator or any similar device, the essential point being that the frequency will be high and the power sumcient to start an are between electrode 36 and 30.

As an illustration of a practical frequency but one to which I do not wish to be limited I might mention 500,000 cycles per second. Much higher frequencies are satisfactory and in some cases preferable.

terminals H.

Other-obvious means of providing a high frequency source to cause a discharge between electrodes I! and are of course well known and may be used.

One of the objects which I am endeavoring to obtain by the circuit disclosed in Figure IV is to provide for ignition" or starting a high frequency high voltage discharge between electrodes and 36 and arrange the circuit in such a manner that the 60 cycle or other commercial frequency circuit which supplies the main power will sustain the are once it is started by the high frequency circuit.

It will be apparent that ordinary commercial frequencies provldea much less expensive source of.power than high frequency circuits but it is simpler to momentarily start the are by a high voltage from frequency circuit with a limited amount of power available.

The choke coils 92 and 93 prevent the high frequency high voltage current from passing into the power circuit and also prevent the low impedance of the power circuit (at Q0 cycles) from diverting current which is required to pass across the arc.

The condensers 96 and a1 serve to reduce the amount of power from the 60 cycle circuit passing into the high frequency circuit to negligible proportions.

It will of course be evident that while the drawings show a symmetrical arrangement of chokes condensors, innumerable other circuits accomplishing similar results will be familiar to those skilled in this art..

It is of interest to consider that the production of heat commercially inoperations of this type at relatively low pressures by means of an electric'arc is a relatively new development and 10 one which involves new factors not found in present are furnaces.

Because of the fact that the pressures are very low and that atmospheric oxygen must be eliminated in all gases it is undesirable to have movable electrodes as in the case of most are fur-' naces in use today. Since the electrodes are relatively fixed and difficult to move, it then becomes advisable to provide means for starting the arc without moving the electrodes and for adjusting the are conditions for varying pressures, temperatures, and other factors without moving the electrodes. The accomplishment of these results is part of the object of the equipment and process here disclosed. I

.At extremely low pressures it is, relatively difficuit to maintain a stable arc and for this reason provision is made by automatic valve 31 to permit the control and introduction of special with magnesium because of its tendency to com-,

blue with this material. When the current through transformer 8 falls to a predetermined value the relay automatically opens, allowing the inert gas to enter through it.

Of course oxygen or any gases containing oxygen are quite objectionable as they will react with the magnesium vapor.

Gases containing carbon compounds tend to produce a metallic magnesium seriously contaminated with carbon just as gases containing traces of oxygen produce a metal contaminated with oxide. I a

The arc at the low pressures employed is quite diiferent from the arc encountered at ordinary atmospheric pressure. At low pressures the arc is "soft" and expands to cover large volumes. The familiar incandescent spots on the surface of the electrodes are also expanded to cover relatively large surfaces.

This. condition lends itself peculiarly well to heating applications of the type in question because intense localized heat is avoided but high temperatures can of course be obtained by balancing the heat input against the heat consumption so that there is a continual slight excess.

localised high temperatures are eliminated.

As an illustration of conditions when operating at pressuressomewhere. around 29" of mercury, on the barometer column, a voltage of volts across the arcgive an arc of around 4" in length with a current flow of around 25 amps.

This is based on carbon electrodes, 80 cycle current, and having an impedance in series with the are which consumes about 30 volts. Such an arc is extremely stable which is somewhat contrary to conditions ordinarily encountered at atmospheric pressures.

These conditions are mentioned to illustrate the difference between the are at these low pressures and the are which would be obtained at normal atmospheric pressures.

Thedropacrossthesrcwiilvarywith the nature of the gas or vapor through which the arc 'is passing, the temperature in the furnace, and to some extent on the configuration of the interior of the furnace. In general the drop betweentheelectrodesappearstobemadeupofa .surfaceresistanceamounting to 10 or ll volts at each electrode and appearing within a millimeter or less of the electrode.

The balance of the drop is at the rate of from 5 to volts per inch of arc length subject to much variation with conditions as outlined above.

The radiation from the arc itself is relatively small compared with the radiation encountered at atmospheric pressures. The are appears to be able to act as a convection heating medium and material emersed in the arc is heated some- ,what in the manner that it would be heated if emersed in a salt bath.

Of course some of the important advantages of the low pressures aside from the special facility for are heating are the ease of causing such vapor can contact all portions of the mass to be reacted.

I have found that in producing metallic magnesium it is convenient to utilize briquettes which may be pressed into form and composed of the calcined magnesium oxide (or calcined dolemite) intimately mixed with finely ground silicon or ferro-silicon. The low pressure of course makes it feasible to remove the metallic magnesium produced by evaporation so that the metal is carried out from the briquette as a vapor.

Of course the arc during this portion of the reaction is sustained by the ionization of the magnesium vapor. In other words it is a magnesium arc quite largely.

I prefer to grind the magnesium oxide or dolemite and the reducing agent quite finely. It is desirable to have both materials ground finer than 200 mesh although I am able to carry out 'the reaction with very much coarser particles.

sults leaving of course an ash consisting of the iron, silica, calcium oxide, etc.

Of course it is desirable to thoroughly calcine and dehydrate the carbonates of magnesium which are used arid I prefer to carry on the calcining at a relatively low temperature and with a long time in order to produce a product having a relatively open structure which accelerates the reaction. This is in distinction to a thoroughly sintered dense structure which appears io retard the reaction.

Instead of using briquettes or other mixtures containing ferro-siiicon and magnesium oxide I may use mixtures of magnesium oxide with carbon in which case it is necessary to quickly cool the resultant vapors to prevent a reversable reaction which would reform magnesium from the reduced metal vapor.

Another process which may be carried out in the equipment here disclosed is the vaporization of a mixture of metallic magnesium, oil, carbon, and various other impurities. This mixture may be distilled in the equipment I have disclosed thus setting free metallic magnesium and leav- -ing the carbon and 'mineral impurities as a residue.

When heat is applied by the arc in air at low pressure the heat will be radiated to the inner sides of the furnace and will raise the temper- 39 ature of the adjacent briquettes or mixtures so I have found that briquettes which are somewhat porous give a much quicker reaction, and therefore a greater production per furnace, than briquettes which are dense. Thus I have used sodium silicate as a binder for briquettes and find it less desirable than a binder such as starch.

It is of course evident that in any case the briquettes must be thoroughly dehydrated so that tomar'y to introduce a charge of material to be heated.

05 The material y consist of ny of the'well hour the voltage will range from 500 to 1000 volts that the reaction between the reducing agent and the magnesia will begin. This will in the case of ferro-siliccn and magnesia produce vapors of metallic magnesium which will soon build up pressure within the furnace chamber so that the arc is carried largely by the vapors of metallic magnesium. The magnetic valve and reservoir of hydrogen will only be needed on occasion where the pressure within the chamber becomes so low that it is diiiflcult to quickly heat up the interior of the furnace.

'. I have found that the flow of current between the electrodes 35 and 36 will vary with several factors and these characteristics must be considered in the design and operation of my equipment The voltage drop between the electrodes will depend on a constant figure which appears to range from 20 to 30 volts and is dependent largely upon the pressure within the furnace chamber and the nature of the vapor or gas which fills it.

Aside from a somewhat constant voltage drop at the electrodes there will be another consumption of voltage which is proportional to the distance between the ends of the electrodes, that is to say the length of the path of the current fiow.

This voltage will vary with the pressure of the vapor or gas within the chamber, the nature of the vapor or gas, and the amount of current passing. Values ranging between 2 volts and 20 volts per inch have been found under commercial conditions when operating at effective temperatures and with commercial loads.

When operating steadily at normal temperatures and delivering heat to produce metallic magnesium at a rate of 200 or 300 pounds per cooling means such as water pads or water jackets. These are details of construction not essential to the basic invention which I have made.

In order to obtain the maximum production from a given'piece of equipment I have found it an advantage as specified previously to utilize a form of magnesium oxide which is as porous and susceptible to reaction as possible. I have found that the best method of producing such a type of oxide is to reduce the carbonates at a low temperature so low that sintering does not occur. This ordinarily gives an oxide which has many of the characteristics of the so-called caustic burned oxides.

I have found that by using material of this kind, it is possible to greatly accelerate the reaction and to have a higher recovery of metal, and less waste of umeduced oxide. I

Obviously the reaction will take place with fused or sintered oxide but ordinarily with lower rates of recovery and higher temperatures and depreciation of the equipment- One factor which controls the tendency to sinter is the amount of fusible materials or fluxes present in the carbonate. Thus large appreciable amounts of alkali metals, iron, and the various materials which lower the fusion point of the oxide are likely to be detrimental in that they cause sintering at lower temperatures. I therefore prefer to calclne carbonates wherever possible at temperatures and rates such that fusing or sintering of the oxide with the residual im ;is helpful in this respect and I prefer to use small amounts of magnesium chloride which may be added to the briquettes.- Naturally the anhydrous product is to be preferred. .j

I prefer masnesium chloride to sodium chloride for the reason that the sodium chloride has a tendency to clog the pores of the oxide and thus vinsome measure retards the reaction.

I do not wish to be confined to any particular 7 compound as many will be recognized as raising the vapor pressure of the metallic silicon or its compounds. In the claims I class this material nnder the broad heading of a catalyst.

Having now fully described my invention, what I claim as new and desire to secure by Letters Patent in the United States is:

1. The continuous process of producing magnesium which consists in removing the carbondioxide from magnesium carbonates at a temperature such that the resultant oxide is relatively porous, mixing said calcined carbonate with non-carbonaceous reducing agent and subjecting the mixture under conditions of low pressure to the direct heat radiated from an electric disin magnesium vapors.

3. In the process of producingmagnesium, the steps of producing a porous active non-sintered oxide of magnesium, mixingsaid oxide while in said active state with a noncarbonaceous mate- .rial containing elemental silicon and heating the upper surface of said mixture under conditions of low pressure by passing an electric current through the vapor of the metal being reduced, and

causing the direct radiation of said electric arc to radiate to said mixture, thus heatin the upper surface of said mixture to a higher temperature than the balance: of the mixture.

4. In the process of producing metallic magnesium the step of heating a mixture containing magnesium oxide witha noncarbonaceous reducing agent under conditions of low pressure, by means of direct radiation from an electric discharge supported by magnesium vapor at low pressure, said radiation directly heating the upper surface-of said mixture initially to a higher temperature than the balance of said mixture so that the reaction first takes place in t e upper surface which results in the evolution of magnesium' vapor from the upper surface under such conditions that the bulk of the vapor does not travel through the entire mass of the mixture.

WILLIAMA. DARRAH. 

